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        <ol class="chapter"><li><a href="../index.html"><strong aria-hidden="true">1.</strong> 超过 88 道练习，任点</a></li><li><a href="../low.html"><strong aria-hidden="true">2.</strong> 易</a></li><li><ol class="section"><li><a href="../hello-world/README.zh.html"><strong aria-hidden="true">2.1.</strong> hello world</a></li><li><a href="../gigasecond/README.zh.html"><strong aria-hidden="true">2.2.</strong> 千兆秒 &gt;&lt; Gigasecond)</a></li><li><a href="../leap/README.zh.html"><strong aria-hidden="true">2.3.</strong> 闰年 &gt;&lt; Leap</a></li><li><a href="../raindrops/README.zh.html"><strong aria-hidden="true">2.4.</strong> 雨滴声 &gt;&lt; Raindrops</a></li><li><a href="../reverse-string/README.zh.html"><strong aria-hidden="true">2.5.</strong> 反转字符串 &gt;&lt; Reverse String</a></li><li><a href="../nth-prime/README.zh.html"><strong aria-hidden="true">2.6.</strong> 第 n 个素数 &gt;&lt; Nth Prime</a></li><li><a href="../bob/README.zh.html"><strong aria-hidden="true">2.7.</strong> 迟钝孩子 &gt;&lt; Bob</a></li><li><a href="../beer-song/README.zh.html"><strong aria-hidden="true">2.8.</strong> 啤酒之歌 &gt;&lt; Beer Song</a></li><li><a href="../proverb/README.zh.html"><strong aria-hidden="true">2.9.</strong> 谚语串烧 &gt;&lt; Proverb</a></li><li><a href="../difference-of-squares/README.zh.html"><strong aria-hidden="true">2.10.</strong> 平方差 &gt;&lt; Difference Of Squares</a></li><li><a href="../sum-of-multiples/README.zh.html"><strong aria-hidden="true">2.11.</strong> 倍数之和 &gt;&lt; Sum Of Multiples</a></li><li><a href="../grains/README.zh.html"><strong aria-hidden="true">2.12.</strong> 谷物 &gt;&lt; Grains</a></li><li><a href="../pythagorean-triplet/README.zh.html"><strong aria-hidden="true">2.13.</strong> 勾股数 &gt;&lt; Pythagorean Triplet</a></li><li><a href="../prime-factors/README.zh.html"><strong aria-hidden="true">2.14.</strong> 素数因子 &gt;&lt; Prime Factors</a></li><li><a href="../series/README.zh.html"><strong aria-hidden="true">2.15.</strong> 子串 &gt;&lt; Series</a></li><li><a href="../armstrong-numbers/README.zh.html"><strong aria-hidden="true">2.16.</strong> 水仙花数 &gt;&lt; Armstrong Numbers</a></li><li><a href="../collatz-conjecture/README.zh.html"><strong aria-hidden="true">2.17.</strong> 3n+1 猜想 &gt;&lt; Collatz Conjecture</a></li><li><a href="../diffie-hellman/README.zh.html"><strong aria-hidden="true">2.18.</strong> 迪菲-赫尔曼密钥交换 &gt;&lt; Diffie Hellman</a></li></ol></li><li><a href="../medium.html"><strong aria-hidden="true">3.</strong> 中等</a></li><li><ol class="section"><li><a href="../saddle-points/README.zh.html"><strong aria-hidden="true">3.1.</strong> 鞍点 &gt;&lt; Saddle Points</a></li><li><a href="../isogram/README.zh.html"><strong aria-hidden="true">3.2.</strong> 等值线 &gt;&lt; Isogram</a></li><li><a href="../say/README.zh.html"><strong aria-hidden="true">3.3.</strong> 英文说数字 &gt;&lt; Say</a></li><li><a href="../run-length-encoding/README.zh.html"><strong aria-hidden="true">3.4.</strong> 游程编码 &gt;&lt; Run Length Encoding</a></li><li><a href="../isbn-verifier/README.zh.html"><strong aria-hidden="true">3.5.</strong> 图书编号 &gt;&lt; ISBN Verifier</a></li><li><a href="../perfect-numbers/README.zh.html"><strong aria-hidden="true">3.6.</strong> 数字也能分类 &gt;&lt; Perfect Numbers</a></li><li><a href="../clock/README.zh.html"><strong aria-hidden="true">3.7.</strong> 时钟 &gt;&lt; Clock</a></li><li><a href="../dot-dsl/README.zh.html"><strong aria-hidden="true">3.8.</strong> DOT DSL</a></li><li><a href="../hamming/README.zh.html"><strong aria-hidden="true">3.9.</strong> 汉明距离 &gt;&lt; Hamming</a></li><li><a href="../simple-linked-list/README.zh.html"><strong aria-hidden="true">3.10.</strong> 简单链表 &gt;&lt; Simple Linked List</a></li><li><a href="../pascals-triangle/README.zh.html"><strong aria-hidden="true">3.11.</strong> 杨辉三角形 &gt;&lt; Pascal's Triangle</a></li><li><a href="../scrabble-score/README.zh.html"><strong aria-hidden="true">3.12.</strong> 字母的分数游戏 &gt;&lt; Scrabble Score</a></li><li><a href="../pangram/README.zh.html"><strong aria-hidden="true">3.13.</strong> 全字母句 &gt;&lt; Pangram</a></li><li><a href="../paasio/README.zh.html"><strong aria-hidden="true">3.14.</strong> PaaS-IO-报告 &gt;&lt; Paasio</a></li><li><a href="../nucleotide-count/README.zh.html"><strong aria-hidden="true">3.15.</strong> 核苷酸计数 &gt;&lt; Nucleotide Count</a></li><li><a href="../luhn/README.zh.html"><strong aria-hidden="true">3.16.</strong> 模 10 算法 &gt;&lt; Luhn</a></li><li><a href="../largest-series-product/README.zh.html"><strong aria-hidden="true">3.17.</strong> 最大数字子串乘积 &gt;&lt; Largest Series Product</a></li><li><a href="../word-count/README.zh.html"><strong aria-hidden="true">3.18.</strong> 单词计数 &gt;&lt; Word Count</a></li><li><a href="../atbash-cipher/README.zh.html"><strong aria-hidden="true">3.19.</strong> Atbash 加密 &gt;&lt; Atbash Cipher</a></li><li><a href="../crypto-square/README.zh.html"><strong aria-hidden="true">3.20.</strong> 密码矩形 &gt;&lt; Crypto Square</a></li><li><a href="../rotational-cipher/README.zh.html"><strong aria-hidden="true">3.21.</strong> 旋转密码 &gt;&lt; Rotational Cipher</a></li><li><a href="../simple-cipher/README.zh.html" class="active"><strong aria-hidden="true">3.22.</strong> 简单加密 &gt;&lt; Simple Cipher</a></li><li><a href="../rail-fence-cipher/README.zh.html"><strong aria-hidden="true">3.23.</strong> 栅栏密码 &gt;&lt; Rail Fence Cipher</a></li><li><a href="../etl/README.zh.html"><strong aria-hidden="true">3.24.</strong> ETL</a></li><li><a href="../accumulate/README.zh.html"><strong aria-hidden="true">3.25.</strong> 集合操作 &gt;&lt; Accumulate</a></li><li><a href="../acronym/README.zh.html"><strong aria-hidden="true">3.26.</strong> 术语 &gt;&lt; Acronym</a></li><li><a href="../sieve/README.zh.html"><strong aria-hidden="true">3.27.</strong> 素数筛 &gt;&lt; Sieve</a></li><li><a href="../rna-transcription/README.zh.html"><strong aria-hidden="true">3.28.</strong> RNA 转录 &gt;&lt; RNA Transcription</a></li><li><a href="../triangle/README.zh.html"><strong aria-hidden="true">3.29.</strong> 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                    <a class="header" href="#simple-cipher" id="simple-cipher"><h1>Simple Cipher</h1></a>
<a class="header" href="#1-readme" id="1-readme"><h2>1. Readme</h2></a>
<a class="header" href="#简单密码" id="简单密码"><h1>简单密码</h1></a>
<p>实现一个简单的移位密码,像 Caesar 和，一个更安全的替换密码.</p>
<a class="header" href="#步骤-1" id="步骤-1"><h2>步骤 1</h2></a>
<p>“如果他有什么秘密要说的话，他就是用密码写的，也就是说，通过改变字母表的字母顺序，一个字也说不出来。如果有人想破译这些，并理解它们的意思，他必须用字母表中的第四个字母，即 D，代替 A，以及其他的字母。
—Suetonius, Life of Julius Caesar</p>
<p>密码是非常直截了当的算法，使文本不可读，同时仍容易允许破译。他们容易受到许多形式的密码分析，但我们幸运的是，我们的小姐妹通常不是密码学家。</p>
<p>用 Caesar密码，加密来自 Julius Caesar 的消息。现在 Caesar 知道加密方式不是很好，但他还是有个好处: 几乎没有人能读对。所以对于一对夫妇的信件已经足够了，让人们无法识别他们所知道的几个字。</p>
<p>你的任务是创建一个简单的移位密码，就像 Caesar 密码一样。这个图像是 Caesar 密码的一个很好的例子:</p>
<p><img src="https://upload.wikimedia.org/wikipedia/commons/thumb/4/4a/Caesar_cipher_left_shift_of_3.svg/320px-Caesar_cipher_left_shift_of_3.svg.png" alt="Caesar Cipher" /></p>
<p>例如:</p>
<p>将”iamapandabear”作为输入到 <code>encode</code> 函数返回密码”ldpdsdqgdehdu”。(虽不足以让我们的信息在运输过程中，确保高保密性).</p>
<p>当将”ldpdsdqgdehdu”放入<code>decode</code>函数时，它将返回原始的”iamapandabear”，让您的朋友阅读您的原始消息.</p>
<a class="header" href="#步骤-2" id="步骤-2"><h2>步骤 2</h2></a>
<p>移位密码是没有乐趣，直到你的妹妹算了出来后！尝试修改代码,允许我们指定一个密钥(key)，并用作移位距离。这称为代换密码.</p>
<p>下面是一个例子:</p>
<p>给定密钥”aaaaaaaaaaaaaaaaaaa”，对字符串”iamapandabar”进行编码将返回原来的”iamapandable”.</p>
<p>给定密钥”ddddddddddddd”，编码我们的字符串”iamapandabore”会返回疑惑的”ldpdsdqgdhdu”.</p>
<p>在上面的示例中,我们为键值设置了 <code>a=0</code>。因此,当明文添加到密钥时,我们最终得到相同的消息。所以”aaaa”不是一个理想的密钥。但是,如果我们把密钥设置为<code>dddd</code>,我们将得到与 caesar 密码相同的东西.</p>
<a class="header" href="#步骤-3" id="步骤-3"><h2>步骤 3</h2></a>
<p>任何密码中最薄弱的环节都是人。让我们通过提供随机性，并确保密钥仅包含小写字母，来使替换密码更具容错性.</p>
<p>如果有人根本不提交密钥,则生成一个长度至少为 100 个字符的真正随机密钥.</p>
<p>如果提交的密钥不只由小写字母组成,那解决方案应该以适当提醒的方式处理错误.</p>
<a class="header" href="#扩展" id="扩展"><h2>扩展</h2></a>
<p>移位密码通过使文本略微疑惑，达到目的，但易受频率分析的影响。替换密码有助于这一点,但当密钥较短，或空格被保留时,仍然非常脆弱。稍后,你会看到一个解决这个问题的练习”crypto-square”.</p>
<p>如果你想在这一领域走得更远，问题引导你，走进如何以安全的方式交换密钥。看一看<a href="http://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange">维基百科上的 Diffie Hellman</a>对于该方案的最早实现之一.</p>
<a class="header" href="#资源" id="资源"><h2>资源</h2></a>
<p>维基百科的替代密码<a href="http://en.wikipedia.org/wiki/Substitution_cipher">http://en.wikipedia.org/wiki/Substitution_cipher</a></p>
<a class="header" href="#2-开始你的表演" id="2-开始你的表演"><h2>2. 开始你的表演</h2></a>
<pre><pre class="playpen"><code class="language-rust editable">pub fn encode(key: &amp;str, s: &amp;str) -&gt; Option&lt;String&gt; {
   unimplemented!(&quot;Use {} to encode {} using shift cipher&quot;, key, s)
}

pub fn decode(key: &amp;str, s: &amp;str) -&gt; Option&lt;String&gt; {
   unimplemented!(&quot;Use {} to decode {} using shift cipher&quot;, key, s)
}

pub fn encode_random(s: &amp;str) -&gt; (String, String) {
   unimplemented!(
       &quot;Generate random key with only a-z chars and encode {}. Return tuple (key, encoded s)&quot;,
       s
   )
}

</code></pre></pre>
<a class="header" href="#3-测试代码查看" id="3-测试代码查看"><h2>3. 测试代码查看</h2></a>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
use std::collections::HashSet;

const PLAIN_TEXT: &amp;str = &quot;thisismysecret&quot;;
const KEY: &amp;str = &quot;abcdefghij&quot;;

#[test]
fn cipher_can_encode_with_given_key() {
   assert_eq!(encode(KEY, &quot;aaaaaaaaaa&quot;), Some(KEY.to_string()));
}

#[test]
//#[ignore]
fn cipher_can_decode_with_given_key() {
   assert_eq!(decode(KEY, &quot;abcdefghij&quot;), Some(&quot;aaaaaaaaaa&quot;.to_string()));
}

#[test]
//#[ignore]
fn cipher_is_reversible_given_key() {
   assert_eq!(
       decode(KEY, &amp;encode(KEY, PLAIN_TEXT).unwrap()),
       Some(PLAIN_TEXT.to_string())
   );
}

#[test]
//#[ignore]
fn cipher_can_double_shift_encode() {
   let plain_text = &quot;iamapandabear&quot;;
   assert_eq!(
       encode(plain_text, plain_text),
       Some(&quot;qayaeaagaciai&quot;.to_string())
   );
}

#[test]
//#[ignore]
fn cipher_can_wrap_encode() {
   assert_eq!(encode(KEY, &quot;zzzzzzzzzz&quot;), Some(&quot;zabcdefghi&quot;.to_string()));
}

#[test]
//#[ignore]
fn cipher_can_encode_a_message_that_is_shorter_than_the_key() {
   assert_eq!(encode(KEY, &quot;aaaaa&quot;), Some(&quot;abcde&quot;.to_string()));
}

#[test]
//#[ignore]
fn cipher_can_decode_a_message_that_is_shorter_than_the_key() {
   assert_eq!(decode(KEY, &quot;abcde&quot;), Some(&quot;aaaaa&quot;.to_string()));
}

#[test]
//#[ignore]
fn encode_returns_none_with_an_all_caps_key() {
   let key = &quot;ABCDEF&quot;;
   assert_eq!(encode(key, PLAIN_TEXT), None);
}

#[test]
//#[ignore]
fn encode_returns_none_with_an_any_caps_key() {
   let key = &quot;abcdEFg&quot;;
   assert_eq!(encode(key, PLAIN_TEXT), None);
}

#[test]
//#[ignore]
fn encode_returns_none_with_numeric_key() {
   let key = &quot;12345&quot;;
   assert_eq!(encode(key, PLAIN_TEXT), None);
}

#[test]
//#[ignore]
fn encode_returns_none_with_any_numeric_key() {
   let key = &quot;abcd345ef&quot;;
   assert_eq!(encode(key, PLAIN_TEXT), None);
}

#[test]
//#[ignore]
fn encode_returns_none_with_empty_key() {
   let key = &quot;&quot;;
   assert_eq!(encode(key, PLAIN_TEXT), None);
}

#[test]
//#[ignore]
fn decode_returns_none_with_an_all_caps_key() {
   let key = &quot;ABCDEF&quot;;
   assert_eq!(decode(key, PLAIN_TEXT), None);
}

#[test]
//#[ignore]
fn decode_returns_none_with_an_any_caps_key() {
   let key = &quot;abcdEFg&quot;;
   assert_eq!(decode(key, PLAIN_TEXT), None);
}

#[test]
//#[ignore]
fn decode_returns_none_with_numeric_key() {
   let key = &quot;12345&quot;;
   assert_eq!(decode(key, PLAIN_TEXT), None);
}

#[test]
//#[ignore]
fn decode_returns_none_with_any_numeric_key() {
   let key = &quot;abcd345ef&quot;;
   assert_eq!(decode(key, PLAIN_TEXT), None);
}

#[test]
//#[ignore]
fn decode_returns_none_with_empty_key() {
   let key = &quot;&quot;;
   assert_eq!(decode(key, PLAIN_TEXT), None);
}

#[test]
//#[ignore]
fn encode_random_uses_key_made_of_letters() {
   let (k, _) = encode_random(PLAIN_TEXT);
   assert!(k.chars().all(|c| c.is_ascii_lowercase()));
}

#[test]
//#[ignore]
fn encode_random_uses_key_of_100_characters_or_more() {
   let (k, _) = encode_random(PLAIN_TEXT);
   assert!(k.len() &gt;= 100);
}

#[test]
//#[ignore]
fn encode_random_uses_randomly_generated_key() {
   let mut keys = HashSet::new();
   let trials = 100;
   for _ in 0..trials {
       keys.insert(encode_random(PLAIN_TEXT).0);
   }
   assert_eq!(keys.len(), trials);
}

#[test]
//#[ignore]
fn encode_random_can_encode() {
   let (k, encoded) = encode_random(&quot;aaaaaaaaaa&quot;);
   assert_eq!(encoded, k.split_at(10).0);
}

#[test]
//#[ignore]
fn encode_random_can_decode() {
   let (k, _) = encode_random(&quot;aaaaaaaaaa&quot;);
   assert_eq!(decode(&amp;k, k.split_at(10).0), Some(&quot;aaaaaaaaaa&quot;.to_string()));
}

#[test]
//#[ignore]
fn encode_random_is_reversible() {
   let (k, encoded) = encode_random(PLAIN_TEXT);
   assert_eq!(decode(&amp;k, &amp;encoded), Some(PLAIN_TEXT.to_string()));
}

#}</code></pre></pre>
<a class="header" href="#4-答案" id="4-答案"><h2>4. 答案</h2></a>
<p><details></p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
extern crate rand;
use rand::Rng;

pub fn encode_random(s: &amp;str) -&gt; (String, String) {
   let mut r = rand::thread_rng();
   let mut key = String::new();
   for _ in 0..100 {
       key.push(char::from('a' as u8 + r.gen_range(0, 26)));
   }
   let encoded = encode(&amp;key, s);
   (key, encoded.unwrap())
}

pub fn encode(key: &amp;str, s: &amp;str) -&gt; Option&lt;String&gt; {
   shift(key, s, 1)
}

pub fn decode(key: &amp;str, s: &amp;str) -&gt; Option&lt;String&gt; {
   shift(key, s, -1)
}

fn shift(key: &amp;str, s: &amp;str, dir: i8) -&gt; Option&lt;String&gt; {
   if key.is_empty() {
       return None;
   }
   let mut o = String::new();
   let mut i = 0;
   let mut key_arr = Vec::new();
   for c in key.chars() {
       if !c.is_ascii_lowercase() {
           return None;
       }
       key_arr.push(c);
   }
   for c in s.chars() {
       let shift = key_arr[i % key_arr.len()] as i8 - 'a' as i8;
       let n = ((c as i8 - 'a' as i8 + dir * shift) % 26 + 26) % 26;
       o.push(char::from('a' as u8 + n as u8));
       i += 1;
   }
   Some(o)
}

#}</code></pre></pre>
<p></details></p>
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